If he had felt he redeemed himself, it seems as though he would stop running. But again, Shane knew throughout his time with the family that he was only 'playing farmer' as he put it. Part of his redeemption may have been doing that one good thing, that would put his sins behind him, and then moving on with his life.
I am not arguing your poing (a good one too!), just bringing up a possibliity
I like your point about doing the one good thing to put his sins behind, but I can't help but think that there may be more. I don't think it is really important to the point of the book though, it is just a question I had in the back of mind as I was reading. There were definitely positive lessons to take from the book.